Abstract

Grasslands occupy significant land area and account for a large proportion of the global soil carbon stocks, yet the direct effects of grazing and genotypic composition on relationships between shoot and root production are poorly resolved. This lack of understanding hinders the development of models for predicting root production in managed grasslands, a critical variable for determining soil carbon stocks. We quantified the effects of season-long defoliation treatments on both shoot and root production across 4 cultivars of a widely planted pasture grass species (Paspalum notatum Flüggé) in a common garden setting in South Florida, USA. We found that infrequently applied (4 weekly) severe defoliation (to 5 cm) substantially enhanced shoot production for all cultivars, while severe defoliation reduced root production across cultivars, regardless of frequency. Overall, there was no significant relationship between shoot and root production. Our results showed that above-ground and below-ground productivity are only weakly coupled, suggesting caution against use of simple above-ground proxies to predict variations in root production in grasslands. More broadly, our results demonstrated that improved modeling and management of grasslands for below-ground ecosystem services, including soil carbon sequestration/stocks, must account for intraspecific genetic variation and responses to defoliation management.

Highlights

  • Grassland ecosystems occupy more than a fifth of earth’s land area and account for a large proportion of the global soil organic carbon (SOC) stocks (Scurlock and Hall 1998; Lal 2010)

  • Since below-ground production may be the largest component of total net primary productivity (NPP) for many grasslands (Gill et al 2002; Hui and Jackson 2006), determining how grazing affects root production will help to predict when grassland ecosystems will behave as carbon sinks, and whether grazing is likely to promote or inhibit carbon sequestration services

  • Root carbon inputs may constitute a disproportionate amount of the total SOC stock compared with shoot carbon (Rasse et al 2005; Poirier et al 2018; Sokol et al 2019), and are especially critical in grassland ecosystems where above-ground tissue is susceptible to frequent removal by fire and grazing (Johnson and Matchett 2001)

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Summary

Introduction

Grassland ecosystems occupy more than a fifth of earth’s land area and account for a large proportion of the global soil organic carbon (SOC) stocks (Scurlock and Hall 1998; Lal 2010). Laboratory and mesocosm studies have found that frequent grazing/defoliation leads to declines in standing root biomass over the long term (Bardgett et al 1998), whereas a global synthesis of data comparing grazed and ungrazed grasslands found a mix of positive and negative effects on standing root biomass (Milchunas and Lauenroth 1993). Overall, this discordance suggests that variations in plant composition, underlying environmental factors, grazing severity or some combination of these factors significantly mediates the effects of grazing on root production

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